Effect of C7+ Properties on Equation-of-State Predictions

Abstract

Abstract This work studies the effect of heptanes-plus (C7+) characterization on equation-of-state (EOS) predictions. Both the Peng-Robinson EOS (PREOS) and a modified Redlich-Kwong EOS are used. Several characterization schemes found in the literature are used in the study. Six reservoir fluids with extended C7+ analysis have been chosen, representing a broad range of compositions and PVT data. First, the study deals with methods for estimating specific gravities and boiling points of petroleum fractions, two properties usually required by critical property correlations. EOS predictions arc made by using critical properties based on specific gravities and boiling points measured experimentally and estimated with correlations. Next, a review of critical-property correlations is given, including a review of commonly used correlations. Four correlations are chosen to study the effect of critical properties on EOS predictions. Results indicate that relatively small differences in critical properties and acentric factor can result in significant differences in EOS predictions. Finally, a study is made of the effect that adjustments in C7+ critical properties Have on EOS predictions; adjustments of EOS constants (Ωa and Ωb) and binary interaction coefficients also are studied. The influence of individual and combined adjustments to C7+ properties are illustrated graphically. They provide qualitative guidelines that can be used when matching experimental PVT data with an EOS.